This invention relates to the prediction of microbursts and windshear which can accompany thunderstorms.
One of the more spectacular of atmospheric events is a thunderstorm. In a thundercloud, charges become separated until the potential difference within the cloud or between the cloud and ground exceeds 100 million volts. When the potentials become that high for cloud-to-ground lighting (CG), a relatively low current discharge occurs, called a leader. The leader ionizes a small irregular path of air in a series of rapid steps. The leader provides the higher conductivity path that the return stroke, the luminous upward traveling discharge from the earth usually perceived as lightning, will travel during a discharge.
Each CG lightning flash may actually be composed of several strokes. Each stroke results in a peak current flow of typically more than 20,000 amps. The passage of this amount of current fully ionizes the air in the lightning channel and results in the production of light and radio frequency (RF) electromagnetic waves. The RF radiation produced in a storm is both vertically and horizontally polarized and has a wide-band frequency distribution.
Not all lightning occurs between earth and ground. The majority of lightning is intra cloud (IC) or between clouds. Interferometric measurements have shown that the majority of discharges take place within a cloud. These discharges are characterized by a pattern of hundreds of RF point sources which propagate horizontally within the cloud and only infrequently turn toward earth to become cloud to ground discharges. A proportionately larger fraction of the energy in RF emissions from IC discharges is in the high, very high (VHF) and ultra high (UHF) frequencies while more of the energy in CG discharges is in the low (LF) and very low (VLF) frequencies.
Although the mechanism by which electric charges are separated in a thundercloud is not completely understood, it is known that the charge separation is accompanied by vertical movements of air. During the initial formation of the thundercloud there is an upward movement of the air mass, an updraft, from the lower part to the upper part of the developing cloud.
The updraft is presumed to contribute to the separation of charge by carrying charged ice particles into the updraft region. These ice particles absorb moisture from the air and grow until they are too heavy to be supported. When that happens, they begin to fall, and continue to grow by sweeping smaller particles from the air column.
As the larger particles fall, they entrain nearby air into a downdraft, which pulls dryer air from outside the falling air column into the column. The dry air causes the ice particles to sublimate, which results in the cooling of the falling air column. As the air cools, it becomes denser and falls faster. A considerable mass of solid and liquid water also may be in the downdraft.
If the falling column of air passes through the 0.degree. C. isotherm toward warmer temperature, the ice particles will melt. The phase change from ice to water will further cool the air, resulting in the additional loss of buoyancy. This loss further increases the air column's downward acceleration. This downward moving column of air, typically 0.4-4 km in diameter and moving in excess of 3-4 m/sec, is called a microburst.
When the microburst approaches the ground, the column of air is deflected and moves along the earth s surface. This can be similar to the way water from a hose sprays radially when the hose is pointed at the ground. Because of this divergence of the air column near the ground, the direction of the air flow near the ground level will appear to vary as one moves vertically over a relatively short distance. This abrupt change in wind velocity with altitude is called windshear.
Both microbursts and windshear are of more than academic interest because of the dangers they pose to aircraft. If an aircraft, taking off or landing, passes into a microburst, it is exposed to a downward acceleration at a time when there may be insufficient altitude for the pilot to avoid crashing.
Windshear poses a similar hazard. The lift provided by an aircraft wing is determined by the velocity of air passing across the wing, i.e., the relative velocity of the aircraft through the air. If the relative velocity decreases past a critical value, the air begins to separate from the wing surface and the aircraft loses the lift necessary to remain airborne. This loss of lift is termed stalling. An aircraft which is taking off or landing is operating just above the velocity at which it stalls. If the wind velocity relative to the ground suddenly decreases or reverses direction, the aircraft may find that its relative airspeed, which was greater than its stall velocity, is now below its stall velocity, and that it has insufficient lift to remain airborne.
This can happen when an aircraft takes off or lands within a region of wind shear. For example, although the wind velocity may initially be such that the relative airspeed is greater than the stall speed of the aircraft, just a short vertical distance away, the aircraft may move into a region where the wind direction reverses, causing the wind speed over the wing to fall below stall velocity. Since this usually happens near the ground, the pilot may not be able to increase the speed of the aircraft quickly enough to regain lift before the aircraft loses what little altitude it has.
Particularly susceptible to these problems are the large jet aircraft used by the airline industry. Because these airplanes are so large, and because a jet engine requires a substantial amount of time to build up trust, a pilot may not be able to accelerate the airplane quickly enough to avoid crashing if the plane stalls because of windshear or is forced downward because of microburst downdrafts and mass loading of the airframe by rain. Over the last few years several airline disasters have been attributed to windshear and microbursts.
Several systems have been invented to alert ground personnel of the existence of potentially dangerous wind variations that would be hazardous to aircrafts which are landing or taking off. One of the systems consists of a two dimensional horizontal array of wind direction and velocity indicators located on the ground at an airport. When a microburst occurs, different sensors in the array indicate a different wind direction and speed at the same time. Although it is possible to infer windshear with this system, only a relatively small region can be monitored. Further, the system can only indicate that a microburst is occurring and can not predict when or where microbursts or windshear will occur.
In an effort to increase the size of the region monitored for windshear and expand coverage into the third dimension (height), Doppler radar is used. Doppler radar measures the velocity component of particles of dust or rain in the direction toward or away from the radar antenna. The principle is that under windshear, a small region of space will appear to have particles moving toward or away from the antenna at different velocities as a function of height. Unfortunately, nothing of the particles moving perpendicular to the radar beam can be determined. Therefore, microburst downdrafts cannot be measured by Doppler radar scanning close to the horizontal plane, which is its normal mode of operation. Doppler radar will detect microbursts only if the radar antenna is pointed above the horizontal so that the downward motion becomes a sufficiently large radial component relative to the antenna. Doppler radar can also detect windshear after the microburst air turns horizontal near the ground. However, if there are no particles, such as when the rain evaporated below cloudbase in the "dry" thunderstorms in clean air which occur in parts of the country, there may be no windshear detected. Again, Doppler radar is mostly useful as an indicator of the presence of windshear, and not as a predictor of its occurrence.
Other devices have been developed to make use of the electrical discharge which occurs to detect the presence of thunderstorm activity. The devices do not measure microburst/windshear and windshear directly but simply warn of the presence of thunderstorms and therefore the possibility of windshear. U.S. Pat. No. 4,023,408 discloses a device which measures the direction to CG and IC discharges and estimates their distance by determining the intensity of the strength of the RF signals generated by the discharges. The assumption is made that the higher the signal strength, the closer the discharge. This device has inherent inaccuracy because it will regard a weak discharge as being further away than a strong discharge at the same distance.
Another device (U.S. Pat. No. 4,672,305) measures the ratio of the electric to magnetic field of the discharge to determining the distance to the discharge if the discharge is within 20 nautical miles, and the ratio of the magnetic fields at two frequencies if the discharge is greater than 20 nautical miles from the receiver. Again this device can only estimate the distance of the lightning flash. Both these lightning mapping systems detect IC discharges, but because of the elongated nature of the radiator, and variable polarization and intensity of the discharges, they cannot resolve the position of the discharge with much accuracy (many miles or tens of miles,-much coarser than the 0.5-2 mile scale of the thunderstorm up and down-drafts).
Lightning interferometers are capable of resolving the IC lightning discharge into a series of radiators. Such devices allow the electrical discharge within the cloud to be mapped with high spatial (0.1 mile) and temporal (msec) resolution and a detailed description of the lightning discharge to be obtained.
None of these devices actually measures or predicts microburst/windshear. The present invention provides a warning to a high degree of certainty that a microburst/windshear is about to occur and its location, allowing pilots ample time to make an appropriate response.